Abstract: A method was established for the simultaneous determination of Cr, Cu, As, Cd by inductively coupled plasma mass spectrometry (ICP-MS) after diluting seawater samples by 10 times. The method uses self-made standard seawater as the matrix matching solution, He gas as the collision gas, and ¹¹⁵In as the internal standard substance. The linear correlation coefficients of Cr, Cu, As and Cd were above 0.9990 in the concentration range of 0 ~ 0.200 μg/L. The detection limit of theses metals was 0.049 μg/L(Cr), 0.027 μg/L(Cu), nd Cd are above 0.9990 in the concentration range of 0 ~ 0.200 μg/L. The detection limit of theses metals was 0.049 μg/L(Cr), 0.027 μg/L(Cu), 0.083 μg/L (As), 0.009 μg/L (Cd). The recovery range of NASS-6 and actual seawater samples were between 92.1% and 106.8%. The detection limit, accuracy, precision and signal long-term stability of this method can meet the requirements of Chinese specification for marine monitoring. This method has been used in the analysis of seawater samples in Weizhou island, Guangxi. The results show a small deviation compared with the FAAS method, so it can be used as the conventional detection method of trace metals in seawater, such as Cr, Cu, As, Cd.